It's about using their client and thereby supporting Steam as a platform - which will, in turn, increase awareness and popularity of it.

Ah, I thought you meant financial support. Yes, being forced to use it for certain games increases its popularity and acceptance, which is why Valve carefully manipulated that outcome. They also do this through things like that shitty free game they gave away and all their free DLC.

It amuses me that people think they're so pro-customer when really they have a selfish and monetary motive behind every single thing they do.